Apple could already be working on a way to make the next iPhone even thinner than its predecessor.

Its latest patent, which was issued by the US Patent and Trademark Office on Tuesday and discovered by AppleInsider, describes a smaller "D-shaped connector" that could replace the standard 3.5mm headphone jack in the next iPhone.

The headphone plug could end up being just two millimetres in diameter.

The new D-shaped plug is flat along one side, so it could only be inserted correctly into a matching D-shaped cavity on the iPhone. That D-shaped hole could be magnetised to capture the headphone plug, or contain a spring-loaded mechanism to retain it.

Here's the two millimeter D-shaped headphone plug Apple just patented:

USPTO

Which would fit into a D-shaped headphone jack of the same size:

USPTO

The iPhone 6S is Apple's thinnest model yet, at just 6.8 millimetres thick. Even before the release of the iPhone 6S, though, rumours started flying that Apple's next release, the iPhone 7, would be between 6 and 6.5 millimetres thick.

The report, which came from KGI Securities analyst Ming-Chi Kuo (who is known to be a reliable source on Apple), put the iPhone 7's thickness in line with the iPod touch or iPad Air 2, both of which are 6.1 millimetres thick. But the company is well-known for patenting products without creating them, so there's no guarantee that this new headphone port will make an appearance in the iPhone 7.